internal/proxy: don't report NotFound errors
internal/proxy: don't report NotFound errors

Change-Id: Ia1e93c0a9861a0de0176c485c69c7a312ade4abf
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/347557
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite